Need advice on solar powering with LiFePo4 battery

[NeverDie]

@willemx said in Need advice on solar powering with LiFePo4 battery:

my nodes only send a few dozen messages per day and are in deep sleep 99.9% of the time, so I do not need much power. I don't expect battery temperature to be an issue in my setup (I live in the Netherlands, very moderate climate)

Those requirements could be met quite simply with an LDO, diode, and a supercap. For a cheap (but not the best) solution, you could consider just an LDO for charge termination plus a diode on your Lifep04. It would not guard against overdischarge though. A supercap doesn't need to worry about overdischarge.

I just now looked at a lifep04 battery on amazon.com with built in high and low voltage cut-off, but the low cutoff was so low (2v) that I wouldn't recommend it. Nonetheless, the entire thing (battery with built-in protection) costs only around $1.80, pro rata.

I don't see that the TP5000 has an undervoltage cutoff.... Not sure if it does or not. Depending on what kind of longevity/cycle-life you need to guarantee, you may or may not care about that.

[SuperNinja]

here is an example of realization which works for me (I have an RFM69).
I transmit temperature, humidity and pressure atmosphere all 30s and sleep. (BMP280 SHT30)
the TP4056: charges and protects the LI-ION against overvoltage and undervoltage (approx 2.7v)
There is surely to criticize or optimize the scheme: power of the solar panel, the battery, the buck Boost etc.
But in the state, it can transmit for 4 days of rain without sunshine (after a full charge in the sun).

[NeverDie]

@SuperNinja May not work on the OP's LifeP04, so I guess it depends on whether the OP wants to stick with a LifeP04 or change battery chemistry.

[skywatch]

Another thing is that most lithium batteries require a constant current charge. Every time a cloud passes by your solar panel output will drop considerably.

I get around this by charging lead acid from solar and lithium from the lead acid. Not the most efficient way, but the safest and kindest to the batteries.

[willemx]

@SuperNinja : Nice 'schematic' !
I had not thought of a buck/boost converter, but this might also be a good idea in my setup (we just had a full week of almost non-stop rain and drizzle).
I think I will stick with the LiFePo4 / TP5000. I guess the switch mode TP5000 should be more efficient than the linear TP4056.
My next target will be designing a 3d printable weather proof case and than I will do some actual field testing. I am a bit worried about the stamina of LiFePo4's in cold weather conditions. Winter is coming...

[SuperNinja]

@NeverDie yes the TP4056 is only valid for li-ion. The 18650 you can find so everywhere in new or "second hand" (laptop battery, hand vacuum cleaner ...) At first it was a cheap weather test, it has been in the garden for 4 years lol
I just took it apart to integrate it into MYSENSORS with RFM69.
@willemx , thanks i wanted to share something that works ;) , Indeed, lifepo4 is more attractive with its 3.2 -3.4v and the TP5000 with a negative point: a resumption of charge at 3.5v which seems a little high according to the testers.

[NeverDie]

@willemx

Remembering now, the undervoltage and overvoltage protection could both be put under the MCU's control, in which case you could get by with just a diode, or equivalent. It wouldn't be as failsafe in the sense that an unforeseen bug in your code might defeat it, but it could be done. In the end, I'm not sure that the chance of encountering a failure in software is necessarily any higher than that of encountering a failure due to defective hardware.

Alternatively, you could put two cells in series, and in principle that would eliminate any chance of overcharging. With that configuration, if you used a 5v solar panel instead of a 6v, then you'd also eliminate any chance of burning out the atmega328p and wouldn't need extra hardware to ensure against that.

For that matter, you could stick with one cell and just use a 3.5v solar panel. With just that one stroke you wouldn't need anything extra to guard against overcharging the battery or over-volting the atmega328p.

That's what makes embedding solar such a fun problem--depending on the use case, there are such a large number of different ways to solve it. So many that the real game is in comparing the trade-offs.

[NaCl]

@willemx how is your project was going?
I found your post because I like to realize something similar with the LifePo4 and the TP5000.
Did you have some problems or did it work as expected?

Greetings
NaCl

[willemx]

@NaCl
No, it did not work as expected. It could not provide enough power for my application. I ended up buying a more powerful setup (10W solar panel + included controller/BMS) with 4 Li-Ion cells and a 3V3 step down converter.

[TerkumOsber]

LiFePO4 batteries are a great choice because they have a long life cycle and are much safer than other lithium-based batteries. However, you need a solar charge controller that’s compatible with LiFePO4 to prevent overcharging and maintain efficiency. A good MPPT controller can significantly boost energy harvest from your panels. Also, ensure your solar panel output matches your power needs. If you're in solar ireland, you might find some local companies that offer tailored solutions for maximizing solar efficiency in lower light conditions.